This invention relates to a method of filling a liquid crystal device with a ferroelectric liquid crystal material.
Vacuum filling method is a known method for filling the space between a pair of substrates of a liquid crystal device with a liquid crystal to manufacture a liquid crystal device. In this prior art method, the liquid crystal device is entered in a vacuum chamber at a vacuum together with the liquid crystal material, and then, after the inlet port of the device is caused to sink in the liquid crystal, the pressure is elevated so that the liquid crystal enters the liquid crystal device through the inlet port. One type of this method is described in Japanese application No. sho 60-175192.
However, there are several shortcomings associated with this method. The inlet port of the liquid crystal device is substantially narrow so that the pressure in the liquid crystal device can not be lowered rapidly. Because of this, the liquid crystal device is subjected to the differential pressure between the inside and the outside of the liquid crystal device when the vacuum chamber is evacuated, and therefore tend to be deformed due to the undesirable pressure.
To comply with the shortcoming, it is proposed to mate a pair of substrates firmly to bear the differential pressure and maintain the distance between substrates by making use of two or more types of spacers arranged therein. Nevertheless, in some instances the device may be destroyed due to a transient force of about 1 kg/cm.sup.2 which arises during evacuation.
Furthermore, the filling process is generally carried out in a particular phase of liquid crystal. A ferroelectric liquid crystal exhibits its phase transition as the temperature varies, e.g. Cry phase .rarw..fwdarw. SmC phase .rarw..fwdarw. SmA phase .rarw..fwdarw. Iso phase. This phase transition is observed also with a blended liquid crystal. An Iso (isotropic) phase is most suitable for filling process since it evidences a low viscousity. An example of the method is described in Japanese Patent Application No. sho 60-175192.
However, when a blended liquid crystal material composed of several constituent liquid crystals is used, one liquid crystal constituent has a tendency to enter through in advance of another so that, when the space is filled, the liquid crystal material becomes nonuniform and has diverse composition different from the prescribed composition depending on the position. Having carefully investigated this phenomena, the inventors found that the transition temperature of the blended liquid crystal is not observed at a certain definitive level and a mixed phase seems to appear due to differential transition temperatures of the constituent liquid crystals. In other word, it may happen during filling process that one constituent liquid crystal is of Iso phase while another constituent liquid crystal remains in Smc phase. This nonuniformity makes it impossible to drive the liquid crystal material by common driving conditions throughout the device.
The filling of blended liquid crystal materials into the liquid crystal material has been carried out through a plurality of openings provided around the mated substrates of the device as shown in FIG. 1(A) for accelerating the filling. Because of differential viscosities of constituents, the liquid crystal material present at the dashed area 8 (FIG. 1(B)) where the flows of liquid crystal material entering through the openings meet has a different property than the liquid crystal material present at other areas. Orientation dispersion has been observed along the area 8. This nonuniform property of liquid crystal causes disparity of driving performance. In some cases, it may happen that the liquid crystal becomes incapable of displaying at certain areas because of the differential transition temperatures between that positions and the other positions.